What about that firmware upgrade? Panasonic released the DMW-VUZ1 upgrade kit that upgrades the FZ1 firmware to version 2.0, effectively converting the camera into an FZ2. This upgrade comes on an 8MB SD card and is currently sold only in Japan for about ¥6000. A machine translation from Japanese of the upgrade description is available here. According to people who own both, an upgraded FZ1 is functionally equal to an FZ2.

I've asked about this problem many times since receiving my FZ10 last November, and have received the same reply you received: "it's not a bug". True, it makes the camera unusable in many situations, but that's not a bug.

The question now is whether the FZ15 and FZ20 work the same way, or allow the viewfinders to remain as bright in Manual mode as in Program mode, even when an external flash is attached. If the problem has been fixed, I'll be buying an FZ20 and selling the FZ10.